In the operation of commercial freezers, refrigerators, air conditioners and other compression-type refrigeration systems, it is desirable to maximize refrigeration capacity while minimizing total energy consumption. Specifically, it is necessary to operate the systems at as low a compression ratio as possible without the loss of capacity that normally occurs when compressor compression ratios are reduced. This is accomplished by suppressing the formation of flash gas. Flash gas is the spontaneous flashing or boiling of liquid refrigerant resulting from pressure losses in refrigeration system liquid refrigerant lines. Various techniques have been developed to eliminate flash gas. However, conventional methods for suppressing flash gas can substantially reduce system efficiency by increasing energy consumption.
FIG. 1 represents a conventional mechanical refrigeration system of the type typically used in a supermarket freezer. Specifically, compressor 10 compresses refrigerant vapor and discharges it through line 20 into condenser 11. Condenser 11 condenses the refrigerant vapors to the liquid state aided by circulating fan 31. The liquid refrigerant next flows through lines 21a and 21b into receiver 12. From receiver 12, the liquid refrigerant flows through line 22 to counter-flow heat exchanger (not shown). After passing through the exchanger, the refrigerant flows via line 23 through thermostatic expansion valve 14. Valve 14 expands the liquid refrigerant to a lower pressure liquid which flows into and through evaporator 15 where it evaporates back into a vapor, absorbing heat. Valve 14 is connected to bulb 16 by capillary tube 30. Bulb 16 throttles valve 14 to regulate temperatures produced in evaporator 15 by the flow of the refrigerant. Passing through evaporator 15, the expanded refrigerant absorbs heat returning to the vapor state aided by circulating fan 32. The refrigerant vapor then returns to compressor 10 through line 24.
In order to keep the refrigerant in a liquid state in the liquid line, the refrigerant pressure is typically maintained at a high level by keeping the refrigerant temperature at condenser 11 at a minimum of approximately 95.degree. F. This minimum condensing temperature maintains pressure levels in receiver 12 and thus the liquid lines 22 and 23 above the flash or boiling point of the refrigerant. At 95.degree. F. this pressure for example would be; 125 PSI for R12 refrigerant, 185 PSI for R22 refrigerant and 185 PSI for R502 refrigerant. These temperature and pressure levels are sufficient to suppress flash gas formation in lines 22 and 23 but the conventional means of maintaining such levels by use of high compressor discharge pressures limit system efficiency.
Various means are used to maintain the temperature and pressure levels stated above. For example, FIG. 1 shows a fan unit 31 connected to sensor 17 in line 21. Controlled by sensor 17, fan unit 31 is responsive to condenser temperature or pressure and cycles on and off to regulate condenser heat dissipation. A pressure responsive bypass valve 18 in condenser output line 21 is also used to maintain pressure levels in receiver 12. Normally, valve 18 is set to enable a free flow of refrigerant from line 21a into line 21b. When the pressure at the output line of condenser 11 drops below a predetermined minimum, valve 18 operates to permit compressed refrigerant vapors from line 20 to flow through bypass line 20a into line 21. The addition to vapor from line 20 into line 21 increases the pressure in receiver 12, line 22 and line 23, thereby suppressing flash gas.
The foregoing system eliminates flash gas, but is energy inefficient. First, maintaining a 95.degree. condenser temperature limits compressor capacity and increases energy consumption. Although the 95.degree. F. temperature level maintains sufficient pressure to avoid flash gas, the resultant elevated pressure in the system produces a back pressure in the condenser which increases compressor work load. The operation of bypass valve 18 also increases back pressure in the condenser. In addition, the release of hot, compressed vapor from line 20 into line 21 by valve 18 increases the refrigerant specific heat in the receiver. The added heat necessitates yet a higher pressure to control flash gas formation and reduces the cooling capacity of the refrigerant, both of which reduce efficiency.
Another approach to suppressing flash gas has been to cool the liquid refrigerant to a temperature substantially below its boiling point. As shown in FIG. 1, a subcooler unit 40 has been used in line 22 for this purpose. However, subcooler units require additional machinery and power, increasing equipment and operating cost and reducing overall operating efficiency.
Other methods for controlling the operation of refrigeration systems are disclosed in U.S. Pat. Nos. 3,742,726 to English, 4,068,494 to Kramer, 3,589,140 to Osborne and 3,988,904 to Ross. For example, Ross discloses the use of an extra compressor to increase the pressure of gaseous refrigerant in the system. The high pressure gaseous refrigerant is then used to force liquid refrigerant through various parts of the system. However, each of these systems is complex and requires extensive purchases of new equipment to retrofit existing system. The expenses involved in the purchase and operation of these methods usually outweigh the savings in power costs.
A more recent method of controlling the formation of flash gas in the liquid line was disclosed in U.S. Pat. No. 4,599,873 by R. Hyde. This method involves the use of a magnetically coupled centrifugal pump placed in the liquid line as seen in FIG. 2. FIG. 2 shows a vapor line 114, a condenser 116, a fan unit 118, a liquid line 120, a receiver 122, a pump 124 and 125, a liquid line 126, a heat exchanger 128, a liquid line 129, a valve 130, a line 131, a control 132, an evaporator 134, a fan unit 138, and a vapor line 144. The purpose of this method is to improve system efficiency by allowing system condensing pressures and temperatures to be reduced as ambient temperatures reduce. The centrifugal pump 124 adds pressure to the liquid line 126 at the point where the liquid line exits from the condenser 122 or receiver without the use of compressor horsepower. This method of using a centrifugal pump to add pressure reduces the amount of flash gas that forms in the liquid line, but does not eliminate it altogether.
Furthermore, examination of the centrifugal pump curve in FIG. 3 shows that as flow increases, the pressure added by the centrifugal pump decreases. However, as flow of refrigerant liquid through the liquid line increases the pressure drop in the liquid line increases by the square of the velocity. This combination of effects as shown in FIG. 4, causes the centrifugal pump to only reduce the formation of flash gas during certain low flow conditions, below point A in FIG. 4. As refrigerant flow increases at high load conditions and the pressure added by the centrifugal pump decreases, the formation of flash gas begins to increase again and system capacity is lost when it is needed most.
Another deficiency of the previously described centrifugal pumping method is that the centrifugal pump is located within the liquid line itself. If the centrifugal pump fails to operate properly for any reason, it becomes an obstruction to flow of refrigerant liquid seriously impairing the operation of the refrigeration system.
The most serious deficiency of the previously described centrifugal pumping method is caused by the state of the refrigerant at the outlet of the condenser 116 or receiver 122. The liquid refrigerant at this location in the system is commonly at or very near the saturation point. Any vapor that forms at the inlet of the centrifugal pump due to incomplete condensation or slight drop in pressure caused by the pump suction or any other reason will cause the centrifugal pump to cavitate or vapor lock and lose prime. This renders the centrifugal pump ineffective until the system is stopped and restarted again, and is very detrimental to pump life and reliability. Due to the constantly varying conditions of operation of the refrigeration system this can occur with great regularity.
A further development pertaining the fields of mechanical air conditioning and refrigeration relating to system optimization is disclosed by U.S. Pat. No. 5,150,580 also by R. Hyde. This development, seen in FIG. 2, involves the transfer of some small amount of liquid refrigerant from the outlet of the centrifugal pump 124 in the liquid line 126 to be injected via conduit 136 into the compressor discharge line 114 by means of the added pressure of the centrifugal pump 124 in the liquid line. The purpose of injection this liquid into the discharge line is to desuperheat the compressor discharge vapors before they reach the condenser to reduce condenser pressure and thereby reduce the compressor discharge pressure. This development is said to improve system efficiency at high ambient temperatures when air conditioning systems work the hardest and system pressures are the highest.
Again, however, as system pressures increase and refrigerant flow rates increase at higher loads, the increased flow rate of refrigerant causes more pressure loss through the condenser. However, this same increased flow rate causes less pressure to be added to the liquid by the centrifugal pump 124 in the liquid line 126. Thus, less liquid is bypassed via conduit 136 into the compressor discharge line and less superheat is eliminated at the time when more reduction is needed. And at some point the pressure loss through the condenser is greater than the pressure added by the centrifugal pump and the effect is lost entirely.
Obviously, there remains a need to provide a stable pressure increase in the liquid line 126 to completely eliminate the formation of flash gas, and likewise a stable pressure increase in the liquid injection line 136 to completely desuperheat the compressor discharge vapors if the improvement in system efficiency is to be realized on a constant and reliable basis regardless of system configuration or refrigerant flow rate or vapor content.
The objectives of the present invention are to:
1) Reliably and constantly increase the pressure in the liquid line to suppress the formation of flash gas without unnecessarily maintaining a high system pressure, and without the possibility of obstructing the flow of refrigerant through the liquid lien.
2) To reliably and constantly inject the correct amount of liquid into the compressor discharge line to maximize the heat transfer in the condenser.
3). To improve the operating efficiency of compression-type refrigeration and air conditioning systems in a constant, controlled and reliable basis regardless of system configuration or refrigerant flow rate.
4). To maximize the refrigeration capacity of refrigeration and air conditioning systems in a constant, controlled and reliable basis regardless of system configuration or refrigerant flow rate.
5). To economically and constantly suppress the formation of flash gas in refrigeration and air conditioning systems without impairing refrigeration capacity and efficiency regardless of system configuration or refrigerant flow rate.
6). To provide a way to inexpensively retrofit existing refrigeration systems to attain the foregoing objects on a reliable and controllable basis regardless of the system configuration or refrigerant flow rate.
7). Further, the previous objects must be met in a way that will not be detrimental to the system in the event of failure of the installed pumping mechanism.
8). Still further, the above objects must be reliably met regardless of the presence of some vapor in the liquid at the inlet of the pumping arrangement since the liquid is at or near saturation.
9). Moreover, the above objects must be met in a way that can be adjusted to satisfy a majority of the wide range of system configurations found in the field.
This invention provides for the refrigeration or air conditioning system to be operated in a way which maximizes energy efficiency and suppresses flash gas formation regardless of system configuration or refrigerant flow rate.
The foregoing and other objects, features, and advantages of the invention will become more readily apparent from the following description of a preferred embodiment, which proceeds with reference to the figures.